Jet and rocket fuel



United States Patent 3,381,046 JET AND RGCKET FUEL Charles A. Cohen, Westfieid, and Clifford W. Muessig,

Roselle, 1 .3., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Aug. 15, 1966, Ser. No. 577,563 16 Claims. (Cl. 260-666) The present invention relates to the production of high energy fuels. More particularly, it concerns fuels used for jet propulsion, either in rockets or in standard jet engines. In its most particular form this case mrtains to a process for producing a jet fuel which is fluid, pumpable at extremely low temperatures and is stable for long periods in storage.

In jet propulsion, the driving force which propells a body forward is in turn produced by the rearward discharge from the body of a jet which is propelled through a restricted orifice. The jet engine, whether used in a rocket or a plane, may be powered by various types of jet engines. Initially, the jet engine may utilize a mixture of surrounding air in the combustion of the fuel which it carries. Alternately, the jet engine may carry its own fuel supply and oxygen or other oxidizing agent necessary for combustion, and therefore function independently of atmospheric oxygen. The three basic types of jet engines are well known; they include rarnjets, turbo-jets and pulse jets. The ramjet is similar to the turbo-jet but differs in that compression in the former is obtained by the ramming effect of the oncoming air, while in the latter,

air is forced into the combustion zone by means of a gas turbine. In the pulse jet engine, compression is obtained by the ramming effect of the oncoming air and the intermittent explosion of fuel which closes the valves in the upstream portion of the combustion zone.

It is well known that in order to produce an effectively functioning jet and rocket engine, a great variety of characteristics must be present in the fuels utilized.

Desiderata in liquid hydrocarbon fuels for jet and rocket engines have been described in detail in various publications, notably in Report No. ASD TR 61-728 issued in May 1962, by Wright-Patterson Air Force Base, Ohio, entitled, Future Air Force Requirements for Hydrocarbon Fuels, by I. R. Fultz and Report No. 650,804 of the Society of Automotive Engineers dated Oct. 4, 1965, entitled, Fuels for Advanced Air-Breathing Weapon Systems by Churchill, Hager, and Zengel both of which are herein incorporated by reference.

Of particular importance are the requirements for low freezing point, high heat of combustion in volumelimited systems, high density, freedom from unsaturation and good stability against deterioration when stored for extended periods of time.

In US. Patent No. 3,200,829 a method is taught for the production of a high energy fuel for jet engines. This fuel is prepared by a two-step hydrogenation of a crude mixture of the dimers of cyclopentadiene (CPD) and methylcyclopentadienes (McCPD). These compounds are in turn obtained by heat treatment of products from the cracking of gas oils, kerosenes or heavy naphthas.

When cyclo'pentadiene is permitted to stand at room temperature or is heated in the range of about 60 to 100 C., it dimerizes by a Diels-Alder mechanism to form a crystalline product melting at about 32 C.

Tricyclo(5,2, 1,0 3,8-Decadiene When hydrogenated catalytically, 1 mol of hydrogen is rapidly absorbed at the 8,9 positions to give a dihydrodicyclopentadiene (DHDCPD) melting at 51 C. and when fully hydrogenated to the 3,4,8,9-tetrahydro dicyclopentadicne (THE-CPD), a crystalline product is obtained melting at about 77 C. While this material would make an excellent jet and rocket fuel by virtue of its stability, freedom from unsaturation, high density and high heat of combustion, (18,880 Btu/1b.), its high melting point and crystalline character precludes its use as a liquid fuel. Outside the special area of solid-fuel rockets all liquid-fuel rockets and jet aircraft employ fuels which must be pumped from a distant fuel storage enclosure to the engine.

It is known in the art that the crystalline dicyclopentadiene and its tetrahydro derivative prepared in the abovedescribed manner, possesses an endo configuration as shown below.

endo Dicyclopentadiene The name endo was used to designate the product in which the cyclopentane ring was turned in toward the cyclohexane ring. Subsequently, it was found that if the dimerization of cyclopentadiene was conducted at higher temperatures than those traditionally utilized in the Diels- Alder mechanism, in the order of about C. or higher, a liquid dimer could be isolated which was shown by oxidative degradation of the dihydro derivative to the bicycloheptane dicarboxylic acid to possess the exo configuration. The exo configuration is as follows:

exo Dicyclopentadiene (DCPD) A similar result could be obtained if endo dicyclopentadiene was heated above 150 C. for an extended period of time. Since only small amounts of the exo configuration are obtained due to the ready formation of higher polymers, these conversions are not practical.

Codirners of cyclopentadiene and 1- or Z-methylcyclopentadiene (MeDCPD) or the dimers and codimers of 1- or 2-methylcyclophentadiene (DMeDCPD) yield in a similar manner the endo product .or hydrogenation and depending on the thermal history of the dimer may yield mixtures of endo or exo products having a range of melting points. The product is as a general rule predominantly endo in nature. Because of this the product was suitable as a fuel for jet engines at moderate elevation levels during atmospheric travel, however, the relatively high freezing point, 30 C., for those products having the highest heat of combustion, present great difficulties in systems designed to use liquid fuels at atmospheric levels higher than 50,000 feet. Since this is almost absolutely essential to modern supersonic jet and rocket travel as well as military needs, it is imperative that extremely low freezing point jet fuels be developed in large, commercial quantities.

In accordance with this invention, it has unexpectedly been discovered that an improved jet fuel may be produced by converting endo THDCPD or its mono or dimethyl derivatives to the exo isomer. These isomers possess extremely low freezing points and will remain liquid under the most extreme conditions ordinarily encountered during jet or rocket travel. This is accomplished by treatment of the endo isomer with appropriate acidic reagents. The exo isomer of THDCPD is still fluid and pumpable at -100 C., has a gross heat of combustion of 150,690 and a net heat of combustion of 142,100 B.t.u./ gal. Since all of the exo ismors of THDCPD, tetra- Acid endo dimer cally below:

hydromethyl dicyclopentadiene (THMeDCPD) and tetrahydro dimethyl dicyclopentadiene (THDMeDCPD) have freezing points below 100 C., it is not necessary and in fact undesirable to separate the individual dimers or recrack unhydrogenated dimers so as to separate individual monomers prior to dimerization, hydrogenation, and isomerization.

Suitable reagents for effecting the isomerization of either the pure endo isomer or mixtures of the endo and exo isomers to substantially all pure exo isomers may be either Bronsted or Lewis acids. In particular, the strong Bronsted acids are preferred; they include sulfuric acid, phosphoric acid, polyphosphoric acid and hydrofluoric acid. However, an extremely wide range of acids may be utilized and typical examples of these acids include boron trifluoride; boron trifiuoride coordination compounds with ethers, phenols, alcohols, lower fatty acids and water; aqueous or anhydrous aromatic sulfonic acids, phosphoric acid on Kieselguhr, strong-acid ion exchange resins such as the sulfonated cross-linked polystyrenes available commercially as Dowex 50x8 and Amberlyst-IS and acidic clays such as Davison DA1 and acid-activated Superfiltrol.

Hydrogenation of the endo DCPD, MeDCPD and DMeDCPD may be readily accomplished With a variety of catalysts such as platinum, palladium, rhodium and Raney nickel or nickel on Kieselguhr, either in the absence or presence of solvents such as benzene, cyclohexane, C -C alcohols, acetic acid, esters such as ethyl acetate and ethers' such as ethyl ether and tetrahydrofuran at temperatures from C. to 225 C. at pressures from 1 atmosphere to about 2000 p.s.i.g., the choice being one of convenience.

Hydrogenation occurs first at the 8,9 positions giving the dihydro derivative which is thermally stable and can be subjected to higher temperatures of up to 250 C.- 300' C. without reverting to monomer or further polymerizing to higher polymers. Accordingly, the dihydro dimer may be thermally treated for a sufiicient length of time to effect maximum conversion to the exo isomer before finally hydrogenating the dihydro dimer to the tetrahydro dimer.

Alternatively, the endo dimer may be hydrogenated without pause to the tetrahydro dimer and then subjected to a heat-soaking treatment at a temperature in excess of 200 C., either in the presence or absence of the hydrogenation catalyst but preferably in an atmosphere substantially devoid of oxygen in order to elfect maximum thermal isomerization to the exo isomer.

Another alternative for obtaining the exo isomer involves treatment of the dimer before hydrogenation with protonating agents, i.e. sulfuric acid, to form exo 8-hyexo tetrahydro dieyclopentadiene In the preferred embodiment of this invention, the hydrogenation is conducted with a nickel catalyst in two stages in which the temperature of the hydrogenation is maintained below about 100 C. until at least 1 mol of hydrogen has been absorbed after which the temperature may be raised to as high as 250 C. in order to fully saturate the dimer. For practical purposes, the hydrogenation may be considered to be substantially complete when the Bromine No. (Cgm./ gm.) drops to 2.5 or less.

The endo THDCPD, endo THMeDCPD and endo DMeTH'DCPD or any mixtures thereof is then contacted with an acidic reagent to effect isomerization to the corresponding exo isomer. The suitability of any specific acidic reagent at a particular temperature, pressure, time and concentration may be readily determined by treatment of a sample of pure endo THDCPD under the conditions selected, followed by separation of the product from the reagent and a determination made of the freezing point or more accurately the retention time in a capillary chromatograph as described below, When the freezing point is used as a measure of the degree of isomerization, it may be assumed for all practical purposes that the isomerization has reached the thermodynamic equilibrium when the freezing point is lowered to '80 C. or below.

In a preferred embodiment of this invention, sulfuric acid, which is typical of the Bronsted acids, is used as the acidic reagent. Preferable range of concentration for this acid is to 104.5% (20% oleum). Treatment of the hydrogenated dimer may be done in several stages of increasing acid strength in either batch or countercurrent flow. For example, endo THDCPD is treated with from 0.1 to 10% by volume of 96 wt. percent sulfuric acid at a temperature in the range of 10 to 70 C. for a period of from 5 minutes to 4 hours, mainly to extract and remove any residual unsaturated material, sulfur compounds and frangible hydrocarbons. Following this, the contacting conditions for the sulfuric acid is varied and about 2 to 25% by volume of 99-100 wt. percent sulfuric acid is utilized at a temperature in the range of 15 C. to 100 C. from about 1 to 30 hours. When chromatographic analysis shows that substantially all of the endo compound has been converted to the exo compound, the hydrocarbon is separated from the acid. This may be accomplished by settling, decantation, centrifugation or the like and after removal of suspended particles of acidic material by electrical coalescers or by treatment with a precipitant such as fullers earth or other siliceous material followed by filtration, the hydrocarbon is stripped or sparged with air or inert gas to remove any dissolved acidic gases such as sulfur-dioxide and finally contacted with or percolated through a sorbent clay. Where the product has been distilled prior to the acid-catalyzed isomerization, there is obtained a finished fuel, after for example, percolation through Attapulgus clay.

Caution should be exercised when using strong Lewis acids such as aluminum chloride and bromide since the isomerization of the endo isomer may proceed beyond the exo isomer to form trans decalin and adamantane or the l-methyl or the 1,3-dimethyl adamantane depending on the starting materials. Detection of these materials may be readily made by capillary chromatography as described below and care should be exercised to prevent this.

The following examples illustrate embodiments of the instant invention.

Example 1 This example describes the preparation and characteristics of endo THDCPD. Commercial dicyclopentadiene having a purity (expressed as monomer) of 95 wt. percent was fractionated under reduced pressure to obtain a heart-cut which solidified on cooling. The product boiled at 170 C., (extrapolated to atmospheric pressure) and melted at 3232.5 C. One kilogram of the distilled diene was charged to a 3-liter rocking-bomb autoclave, 50 grams of Raney nickel (in isopropyl alcohol) added and the bomb contents purged with nitrogen. Hydrogen was admitted at a pressure of 1000 p.s.i.g. and the temperature raised to 35 C. whereupon immediate absorption of hydrogen occurred and the temperature rose to 60 C. When absorption slowed down, the temperature was slowly raised to 100 C. and hydrogenation continued for a total time of 3 hours when no further absorption of hydrogen was noted. On cooling the bomb, a solid, crystalline endo THDCPD was obtained which when separated from the catalyst, had a boiling point of 191-l93 C. at atmospheric pressure and melted at 76.5 77 C.

A sample of the product dissolved in benzene as an internal standard was analyzed by capillary gas chromatography as shown below.

Instrument Barber-Colman. Column 150 ft. capillary. Coating Polypropylene glycol. Flash heater 215 C.

Column heater 125 C.

Cell temperature 210 C.

Ionization source Strontium 90. Carrier gas Argon.

Voltage 1250 volts.

A single peak was obtained having a retention time of 86.7 compared to a retention time for benzene taken as Example 2 This example illustrates the production and characteristics of exo THDCPD,

Five hundred grams of endo THDCPD, prepared as in Example 1 was mixed with 250 grams of 99.5105 sulfuric acid in a stirred reaction vessel maintained at 90i5 C. Some heat of reaction was noted on initial mixing and some oxidation occurred as judged by the odor of S0 from the reaction mixture. Stirring at the above temperature was continued for six hours after which the reaction mixture was poured into a graduated cylinder and allowed to settle overnight. 485 grams of a liquid hydrocarbon layer was decanted from the acid and a portion of the material after clean-up, detailed below, was analyzed as in Example 1 by capillary gas chromatography. Two peaks were obtained in approximately 2:1 ratio; corresponding to retention times for exo THDCPD at 69.5 and unchanged endo THDCPD at 86.7; both were compared to benzene taken as zero.

The partially isomerized product was mixed with 20% by volume of the same acid and stirred at 95i5 C. for 20 hours, the reaction mixture centrifuged to separate the acid from the hydrocarbon, the hydrocarbon layer mixed with 1% wt./vol. of purified diatomaceous earth, filtered under suction and after sparging with nitrogen 6 to remove dissolved S0 was finished by percolating through 5% wt./ vol. of 30-60 mesh Attapulgus clay contained in a filter-tube having a 20:1 height to diameter ratio.

The percolate was bland in odor, water-white, had a boiling point of 185186 C. and was free of unsaturation as determined by bromine absorption and infrared spectroscopy. Analysis of the product before and after distillation by capillary G.C. showed an exo content of about 99% with about 1% of the endo isomer. No trans decalin which has a retention time of 63.1 nor adamantane with a retention time of 83.5 was observed.

A sample of the exo product was stored in a glass container for 48 hours in a bath composed of solid carbon dioxide and acetone at an observed temperature of C. No crystallization of any solid material was observed and the product readily poured when emptied from its container. The following inspections were obtained:

Specific gravity, 60 F./60 F 0.9382

Pounds per gallon 7.813 Kinematic viscosity:

77 F. cs 3.06

0 F. cs 8.665

-60 F. cs 31.9 Freezing point (ASTM D1477) C Below -80 Flash point (closed cup) F.-- 128 Conradson carbon 0.00 Aniline point F +85.0 Heat of combustion:

Gross B.t.u./lb.. 19,282

Net B.t.u./lb. 18,183

Example 3 Commercial dimethyl dicyclopentadiene (DMeDCPD) consisting of dimers and codimers of l-methyl cyclopentadiene and Z-methyl cyclo-pentadiene in a ratio of about 35% of the 1-methyl and 65% of the Z-methyl diene having an overall purity of 96% (expressed as monomer) was fractionated under reduced pressure and a heart-cut taken for hydrogenation. It had a boiling point of 208-210 C. (extrapolated to atmospheric pressure) and a freezing point below -60 C.

While the freezing point and heat of combustion of this material might suggest its suitability as a jet and rocket fuel, it is highly unstable, forming peroxides, higher polymers and gummy autoxidation products. Hydrogenation of the dimethyl dicyclopentadiene to the 8,9 dihydro dicyclopentadiene yields a product boiling at 48-49 C. 2 torr (215218 C. atm.) and stabilizes it against formation of higher polymers but the residual unsaturation in the 3,4 positions makes the dihydro derivative still subject to oxidation and gum formation.

One liter of the heart-cut dimer prepared as above was hydrogenated with Raney nickel catalyst as in Example 1 but a final temperature of 160 C. was used to obtain substantial saturation. Fractionation of the hydrogenated product under a 30 plate column yielded a heartcut fraction of tetrahydro dimethyl dicyclopentadiene (THDMeDCPD) boiling at 212 to 220 C. at atm0s pheric pressure. Analysis showed the following:

Specific gravity, 20/20 C. 0.929

Bromine No. (ASTM D-ll59) 0.6

Freezing point (ASTM D-1477) C 30 Example 4 One hundred parts by volume of THDMeDCPD was reated with 20 parts by volume of 9989:001 wt. percent sulfuric acid at a maximum temperature of C. for 2 hours with good stirring. The hydrocarbon layer and the acid both darkened as the treatment progressed. The reaction mixture was settled, the hydrocarbon layer decanted from the acid and finished by contacting with diatomaceous earth, filtration under vacuum and nitro- 7 gen sparging and clay percolation as in Example 2. A freezing point determination (by ASTM Procedure D 1477) showed a freezing point below 80 C. and zero unsaturation by bromine number.

Alternatively, the acid treated hydrocarbon layer may be neutralized with aqueous or aqueous-alcoholic solutions of alkaline hydroxides, carbonates and the like and dehydrated by stripping with steam, blotter pressing or settling. Variations in the finishing procedures after isomerization will be readily apparent to those having skill in the art of acid-treating hydrocarbons. See for example U.S. Patents 2,348,609 and 2,688,633 which are herein incorporated by reference.

Jet and rocket fuels prepared as above are non-corrosive in contact with metals, free of unsaturation, are readily inhibited against peroxide and gum formation by nominal amounts of phenolic or amine type antioxidants and are stable in storage for extended periods of time.

Although this invention has been described with some degree of particularity, it is intended to be limited only by the attached claims.

What is claimed is:

1. A process for the production of an improved fuel which may be used in jet and rocket engines which comprises:

(a) hydrogenating an endo dimer of an alicyclic conjugated diene selected from the group consisting of cyclopentadiene, methylcyclopentadiene, and mixtures thereof;

(b) contacting said hydrogenated dimer with an acidic reagent thereby converting at least a portion of endo isomer to its exo isomer;

() and separating said exo isomer from said acidic reagent.

2. The process of claim 1 wherein said acidic reagent is a Bronsted acid.

3. The process of claim 1 wherein said acidic reagent is sulfuric acid.

4. The process of claim 1 wherein said acidic reagent is a Lewis acid.

5. A process for producing a fuel applicable to both jet and rocket engines, said fuel having a low freezing point which comprises:

(a) hydrogenating endo dicyclopentadiene substantially to endo tetrahydro dicyclopentadiene;

(b) contacting said hydrogenated endo tetrahydro dicyclopentadiene with a strong acidic reagent at reaction conditions thereby converting substantially all of said endo tetrahydro dicyclopentadiene to exo tetrahydro dicyclopentadiene;

(c) and separating said exo tetrahydro dicyclopentadiene from said acidic reagent thereby recovering a compound having a substantially lower freezing point and a substantially lower melting point relative to said endo tetrahydro dicyclopentadiene.

6. The process of claim 5 wherein endo tetrahydro methyldicyclopentadiene is substituted for endo tetrahydro dicyclopentadiene.

8 7. The process of claim 5 wherein endo tetrahydro dimethyl dicyclopentadiene is substituted for said endo tetrahydro dicyclopentadiene.

8. The process of claim 5 wherein said acidic reagent 5 is selected from the group consisting of sulfuric acid, polyphosphoric acid, orthophosphoric acid, hydrofluoric acid and oleum.

9. The process of claim 5 wherein said acidic reagent is a Lewis acid.

10. The process of claim 5 wherein said acidic reagent is sulfuric acid, said contacting takes place at a temperature of 20 to 100 C. for a period of 5 minutes to 24 hours.

11. A jet and rocket fuel comprising exo tetrahydro dicyclopentadiene.

12. A jet and rocket fuel comprising exo tetrahydro methyldicyclopentadiene.

13. A jet and rocket fuel comprising exo tetrahydro dimethyl dicyclopentadiene.

14. A process for the production of an improved fuel which may be used in jet and rocket engines which comprises:

(a) hydrogenating an endo dimer of an alicyclic conjugated diene selected from the group consisting of cyclopentadiene, methylcyclopentadiene, and mixtures thereof to at least the dihydrogenated derivatives;

(b) thermally treating said dihydrogenated dimer thereby converting at least a portion of endo isomer to its exo isomer;

(c) and hydrogenating the said exo dihydro isomer to the exo tetrahydro isomer.

15. A process for the production of an improved fuel which may be used in jet and rocket engines which comprises:

(a) hydrogenating to substantially complete saturation an endo dimer of an alicyclic conjugated diene selected from the group consisting of cyclopentadiene, methylcyclopentadiene, and mixtures thereof whereby a tetrahydrogenated dimer is formed;

(b) and thermally treating said tetrahydrogenated dimer thereby converting at least a portion of endo isomer to its exo isomer.

16. The process of claim 15 wherein the thermal treatment takes place at a temperature of at least about 200 C.

References Cited UNITED STATES PATENTS 2,712,497 7/1955 Fox et al. 4480 2,765,617 10/1956 Gluesenkamp et al. 44-80 3,002,829 10/ 1961 Kolfenbach et al. 44-80 3,004,384 10/1961 Saunders 44-80 3,167,595 1/1965 Heywood et al. 260-666 3,235,614 2/1966 Fritz et al 260-666 DELBERT E. GANTZ, Primary Examiner.

V. OKEEFE, Assistant Examiner.

REEXAMINATION CERTIFICATE (180th) United States Patent 19 Cohen et al.

[45] Certificate Issued Mar. 27, 1984 [54] JET AND ROCKET FUEL [75] Inventors: Charles A. Cohen, Westfield; Clifford W. Muessig, Roselle, both of NJ.

Esso Research and Engineering Company [73] Assignee:

Reexamination Request:

No. 90/000,115, Nov. 27, 1981 Reexamination Certificate for:

Patent No.: 3,381,046

Issued: Apr. 30, 1968 Appl. No.: 577,563

Filed: Aug. 15, 1966 [51] Int. Cl. C07C 5/03; CO7C 5/22 [52] US. Cl. 585/253; 585/2;

OTHER PUBLICATIONS Ya. M. Paushkin, The Chemical Composition and Properties of Fuels for Jet Propulsion, pp. 71-77, (B. P. Mullins, ed.) (1962; Pergamon Press).

N. A. Ragozin, Jet Propulsion Fuels, pp. 46-53 (B. P. Mullins, ed.) (1961, Pergamon Press).

R. C. Weast; S. M. Selby, Handbook of Chemistry and Physics, P. C-288 (1966, The Chemical Rubber Co.). Schleyer et al., J.A.C.S., vol. 82, pp. 4645-4651 (1960). Bruson et al., J.A.C.S., vol. 67, pp. 723738 (1945). Kirk-Othmer (I), Encyclopedia of Chemical Technology, pp. 282, 285, 286, 292-296 (1st Ed., 2nd Supplement, vol. 1960). Kirk-Othmer (II), Encyclopedia of Chemical Technology, vol. VI, pp. 701, 704 (2nd Ed., 1965).

Primary Examiner-Brian E. Hearn EXEMPLARY CLAIM 14. A process for the production of an improved fuel which may be used in jet and rocket engines which comprises:

(a) hydrogenating an endo dimer of an alicyclic conjugated diene selected from the group consisting of cyclopentadiene, methylcyclopentadiene, and mixtures thereof to at least the dihydrogenated derivatives;

(b) thermally treating said dihydrogenated dimer thereby converting at least a portion of endo isomer to its exo isomer;

(c) and hydrogenating the said exo dihydro isomer to the exo tetrahydro isomer.

AS A RESULT OF REEXAMINATION, IT HAS REEXAMINATION CERTIFICATE BEEN DETERMINED THAT:

ISSUED UNDER 35 The patentability of claim 14 is confirmed.

THE PATENT IS HEREBY AMENDED AS Claims 1-13, 15 and 16, having been finally deter- INDICATED BEL mined to be unpatentable, are cancelled. 

1. A PROCESS FOR THE PRODUCTION OF AN IMPROVED FUEL WHICH MAY BE USED IN JET AND ROCKET ENGINES WHICH COMPRISES: (A) HYDROGENATING AN ENDO DIMER OF AN ALICYCLIC CONJUGATED DIENE SELECTED FROM THE GROUP CONSISTING OF CYCLOPENTADIENE, METHYLCYCLOPENTADIENE, AND MIXTURES THEREOF; (B) CONTACTING SAID HYDROGENATED DIMER WITH AN ACIDIC REAGENT THEREBY CONVERTING AT LEAST A PORTION OF ENDO ISOMER TO ITS EXO ISOMER; (C) AND SEPARATING SAID EXO ISOMER FROM SAID ACIDIC REAGENT. 